1. Field of the Invention
The present invention relates to a polyvinyl alcohol (i.e., a vinyl alcohol-based polymer). More precisely, the invention relates to a silyl group functionalized polyvinyl alcohol which may be readily dissolved in water to form aqueous solutions without the addition of an alkali such as sodium hydroxide or an acid and whose aqueous solutions have good viscosity stability. The binding force of the polymer with inorganic substances is high and films containing a mixture of the polymer with an inorganic substance have excellent water resistance.
The invention also relates to a method for producing the polyvinyl alcohol. The invention further relates to a coating agent that contains the polyvinyl alcohol and to coated products such as inkjet recording materials and thermal recording materials fabricated by applying the coating agent to a substrate.
2. Description of the Related Art
Vinyl alcohol-based polymers such as polyvinyl alcohols (hereinafter abbreviated as PVA) are known water-soluble synthetic polymers that may be used as a starting material for the synthetic fiber vinylon. PVA is also widely used in other fields such as paper processing, fiber processing, adhesives, stabilizers in emulsion polymerization and suspension polymerization, binders for inorganic substances, films, etc. As compared with other synthetic polymers, PVA is especially desirable for its mechanical strength and film-forming ability, and based on such its characteristics PVA is favorably used in clear coating agents that improve the surface characteristics of paper or binders in pigment coating.
Various attempts have been made to modify PVA and expand its use to other applications. One way is to introduce silicon (e.g., in the form of silyl groups) into PVA. A silyl group functionalized PVA provides excellent water-resistance and has excellent reactivity and adhesiveness to inorganic substances. For example, a method is known for producing silyl group functionalized PVA which comprises dissolving a silylating agent such as triethylchlorosilane in an organic solvent then adding a powdery PVA thereto and reacting them with stirring (JP-A 55-164614). However, the method is problematic in that it provides a homogeneously modified product only with difficulty and requires an additional step of reacting the PVA with a silylating agent in addition to the PVA production itself. From the industrial viewpoint, therefore, the method is impracticable.
Other methods of silyl group functionalized PVA production free from these problems have been proposed. For example, one method comprises hydrolyzing a copolymer of a vinyl alkoxysilane such as vinyltriethoxysilane and vinyl acetate (JP-A 50-123189); another method comprises hydrolyzing a copolymer of a silyl group functionalized acrylamide derivative and a vinyl ester such as vinyl acetate (JP-A 58-59203); still another method comprises hydrolyzing a copolymer of a specifically-substituted silyl group functionalized monomer and a vinyl ester (JP-A 58-79003); and still another method comprises hydrolyzing a copolymer of a silyl group functionalized allyl monomer and a vinyl ester (JP-A 58-164604).
However, the silyl group functionalized PVAs obtained in these methods have problems including (a) in preparing an aqueous solution of the silyl group functionalized PVA, some of the silyl group functionalized PVAs do not dissolve in water unless an alkali such as sodium hydroxide or an acid is present; (b) the viscosity stability of the aqueous solution of the silyl group functionalized PVA is not good; (c) the water-resistance of films formed of the aqueous solution of the silyl group functionalized PVA is not good; and (d) when films that contain the silyl group functionalized PVA and an inorganic substance are formed, they can not simultaneously provide sufficient binding force between the polymer and the inorganic substance and water-resistance.
An ionic hydrophilic group functionalized, silyl group functionalized PVA has been proposed (JP-A 59-182803); and it is reported that a PVA having a silanol group in its side branches has a strong interactivity with inorganic substances (Journal of the Chemical Society of Japan, 1994, (4), 365-370). However, even these modified PVAs do not solve the above-mentioned problems (a) to (d).
An object of the invention is to provide a silyl group functionalized polyvinyl alcohol which may be readily dissolved in water to form an aqueous solution without adding an alkali such as sodium hydroxide or an acid thereto; whose aqueous solutions have good viscosity stability; films of the aqueous solution have excellent water resistance; the binding force with inorganic substances is high; and films containing a mixture of the polyvinyl alcohol and an inorganic substance have excellent water resistance.
The invention also relates to a method for producing the polyvinyl alcohol. The invention further relates to a coating agent that contains the polyvinyl alcohol, and to coated products such as inkjet recording materials and thermal recording materials fabricated by applying the coating agent to a substrate.